Moisture-dependent multiscale mechanism of freeze-thaw damage in bamboo scrimber

Bamboo scrimber (BS), a sustainable high-strength bio-based composite, shows strong potential for structural applications in cold environments. However, its freeze–thaw durability and the mechanistic role of moisture remain insufficiently understood. This study adopts a multiscale framework integrating mechanical testing, thermo-mechanical analysis, low-field nuclear magnetic resonance, micro-computed tomography, and finite element modeling to investigate moisture-dependent degradation from molecular to macroscopic scale. The results show that high-moisture BS exhibits significantly greater deterioration, with shear strength decreasing by 18.6% after 40 freeze–thaw cycles compared to only 2.3% in air-dry specimens. Thermophysical analyses indicate that the phase transition of free water introduces pronounced volumetric mismatch, generating localized stresses up to 76.7 MPa at resin–bamboo interfaces and vascular bundles. These stresses initiate interfacial microcracking, which subsequently propagates through the cellular network accompanied by pore coalescence and redistribution of water toward less confined states. The degradation follows a two-stage evolution characterized by rapid early structural reorganization and subsequent quasi-stable damage development. These findings highlight the critical role of free water in triggering irreversible freeze–thaw damage under saturated conditions. This work provides a mechanistic basis for improving the durability of bamboo-based composites through moisture control and optimized resin distribution, supporting their reliable application in cold-region structures.